Te01 mode coaxial waveguide system and rotary joint



June 8, 1965 H. s. KEEN 3,188,583

TE MODE COAXIAL WAVEGUIDE SYSTEM AND ROTARY'JOINT Filed May 28. 1963 3 Sheets-Sheet 1 INVENTOR HENRY s. KEEN BY *31' June 8, 1965 H. s. KEEN 3,188,588

TE MODE COAXIAL WAVEGUIDE SYSTEM AND ROTARY JOINT Filed May 28, 1963 5 Sheets-Sheet 2 INVENTOR F/G. 2 HENRY S. KEEN ATTOANH United States Patent 3,188,588 T11 MSDE (IQAXIAL WAVEGEEEE SYSTEM AND ROTARY IGINT Henry S. Keen, Commacls, N.Y., assignor to Cutler- Harnnrer, Inc., Milwaukee, Wis, a corporation of Delaware Filed May 28, 1963, Ser. No. 233,923

7 Claims. (Cl. 33398) This invention relates to improvements in electro-magnetic waveguide systems, and more particularly to devices such as rotary joints for carrying microwave energy at relatively high power levels in the TE coaxial mode. It is known that circular Waveguides operating in the TE mode exhibit decreasing transmission loss with increasing frequency, and can be designed to provide substantially higher efficiency than waveguides operating in their dominant modes. Further, since the TE mode in circular waveguide involves only fields that are angularly uniform, i.e., independent of radial direction from the axis, it is of interest for use in rotary joints, because circular waveguides coupled end to end could be rotated with respect to each other about their axis without variation in phase or amplitude of the energy transferred between them.

Rotary joints and other transmission systems with circular waveguidesin the TE mode have been proposed, and used to some extent. However, substantially all microwave energy sources, and most utilization devices, terminate in rectangular waveguides operating in the dominant TE mode. Accordingly it is generally necessary to provide at least one, and usually two, conversions between the two modes. Prior art devices for this purpose fall into two main categories; oneemploys grids of conductive wires in the circular waveguide, shaped to act as mode converters, with other grids designed as mode filters, and a transition section between the rectangular and circular guides. This type of device is inherently limited to operation at power levels and bandwidths that are small fractions of the capabilities of the associated main waveguides, owing to field concentrations on the grid conductors and resonant spacings of the grids. Mode converters of the other class consist of special transition sections, many wavelengths long, in which the rectangular guide cross section is gradually distorted into a sector of a circle along the length of the section by Widening and curving one narrow wall and narrowing the other narrow wall, at the same time increasing the spacing between the broad walls until, at the'other end of the device, the guide is of circular cross section, At this point the broad wall is unnecessary and can be eliminated. Such transitions are capable of handling substantially the same power and v bandwidth as the associated rectangular waveguides, but are clumsy and expensive. 7 g

In many waveguide installations, for example those used in certain radar systems, it is desirable or necessary to vide a central space forthe feed lines or waveguides going to the upper joints. The mechanical requirements of around-the-mast rotary joints are essentially incompatible with those of circular waveguides using the TE mode and the associatedrnode converters, which necessarily occupy spaces of substantial length along and on the axis of the structure.

The principal object of the present invention is to proice I vide improved rotary joints for waveguide transmission systems; i

More specifically, it is an object of this invention to provide a type or" rotary joint capable of handling high power levels with low loss and with negligible variation in transmission caused by rotation.

Another object is to provide a type of rotary joint that can readily be designed for around-the-mast and stacked joint assemblies.

A corollary object is to provide improved high power broad band devices for conversion between the rectangular TE and the coaxial TE modes, requiring no complex transistion structures and no mode filters.

The foregoing objects are achieved in the practice of this invention by coupling the end of a coaxial waveguide substantially directly to the ends of a plurality of rectangular waveguides disposed with their broad walls parallel to respective equiangular spaced radii of the coaxial waveguide. The rectangular waveguides are manifolded or otherwise coupled equally to a main rectangular waveguide, in such manner that the TB fields in the rectangular guides at the junction with the coaxial guide will approximate respective sectors of the TE iield pattern in the coaxial guide. Simple matching structures at the junction improve the approximation to the extent that there is no appreciable excitation of other modes in the coaxial waveguide, and a minimum of reflected wave from the junction.

The invention will be described with reference to the accompanying drawings, wherein:

FIG. 1 is a perspective view ofa rotary joint illustrating a presently preferred embodiment of the invention; FIG. 2 is another perspective view of a major portion ofthe structure of FIG. 1, partially disassembled. and partially cut away to illustrate internal details; and

FIG. 3 is a plan view of the lower half of the structure of FIG. 1. 7

Referring to FIG. 1, the rotary joint comprises a lower assembly generally designated by the reference character 1, terminating in a main rectangular waveguide 2, and an upper assembly 3, terminating in a main rectangular waveguide 4. The waveguide 2 is adapted to be connected by way of a flange 5 to another rectangular guide, not shown, running to a radar transmitter-receiver system, for example. The guide 4 may be connected to a directive antenna mounted on or above the assembly 3. I i

Mechanically, the upper and lower assemblies 1 and 3 are substantially identical, the upper assembly 3 being supported on a ring bearing arrangement 6 (shown more clearly in FIG; 2 and to be described therewith) for rotation with respect to the lower assembly 1. Referring particularly to the upper assembly 3 as shown in FIG. I,

the main rectangular guide 4 is the common arm of an- E plane junction 7 having side arms 8 and 9. The junction 7 is of conventional design, and includes matching elements, of a type shown more clearly in the section of 'unction 12 at the top of FIG. 3, to match the impedances of the arms 8 and 9, which are electrically in series with each other, to that of a rectangular waveguide of standard proportions connected to the common arm 4'. Arms 8 and 9 are connected to identical branch waveguides 10 and 11, leading to the common arms of respective E plane directed waveguide section 20, open at its lower end. The open ends of sections are secured to a fiat annular conductive plate 21 which is provided with rec- .tangular openings conforming to and registering with the open ends of waveguide sections 20. The outer periphery of the annular plate 21 is secured to one end of a hollow cylindrical conductor 22, and the inner periphery is. secured to a cylindrical conductor 23 disposed coaxially within the conductor 22. Conductors 22 and 23 form the outer and inner conductors respectively of a short length of coaxial waveguide, operating in the TE mode.

Since the lower assembly 1 is essentially a duplicate of the upper assembly 3, its elements are designated in the drawings by the same reference characters as the corresponding elements of the upper assembly, and the plan view of the lower assembly 1 shown in FIG. 3 is the 'same as an inverted plan view of the upper assembly. Referring to FIG. 3, it can be seen that each rectangular aperture in the plate 21 is centrally and symmetrically disposed with respect to a corresponding 45 degree sector of the coaxial waveguide formed by conductors 22 and 23. The radially extending boundaries of one such sector are indicated by dash lines 24.

Matching between the eight open rectangular waveguide ends and the coaxial waveguide is provided by wedge shaped conductive blocks 25 disposed on the plate 21 symmetrically between the rectangular openings. The apex angle of blocks 25 is such as to provide a plane wall surface 26 parallel to the axis of the coaxial waveguide, bisecting the angle between each long side of each opening and the adjacent sector boundary 25. The height of the matching elements 25, i.e. the dimension axially of the coaxial waveguide, is preferably about one quarter guide wavelengthat mid-band.

Returning to FIG. 2, the outer conductor 22 of the coaxial transmission line of the upper assembly 3 is provided near its lower end with a bearing seat 27 adapted to engage the inner surface 28 of a ring bearing 29.

The bearing 29 may be an antifriction device of the four-point angular contact type, for example, to accommodate unbalanced off-axis longitudinal forces as well as thrust and radial loads. The inner race of the bearing 29 is secured to the seat 27 by a retainer ring30 fastened to the seat by a number of screws such as 31.

On the lower assembly 1, a bearing seat 32 at the upper end of outer conductor 22 is adapted to engage the outer surface 33 of the bearing 29. A retainer ring 34 secures the outer race in the seat 32 when fastened thereto by screws such as 35, and completes the joining of the lower and upper assemblies 1 and 3 to form the rotary joint shown in FIG. 1. The lengths of coaxial conductors 22 and 23 are such that a small circumferential gap is left between the ends of the respective coaxial waveguide sections of the assemblies 1 and 3, at both the inner and outer conductors. This gap provides mechanical clearance for relative rotation of the assemblies .but has no effect on the coaxial TE mode wave transcited 180 out of phase with each other, as indicated by the arrows 37 and 38 in FIG. 3. However, left and right bends in the waveguide sections leading to the next series of power dividers 14, 15, 16, 17 are such that the microwave power arriving at the next series of junctions will be in the correct polarity to be additive in circular fashion within the coaxial structure with their elecltric fields directed as indicated by the arrows 41, each providing a respective sectoral element of a TE coaxial mode field.

In a reciprocal manner, TE waves arriving at the eight apertures of the upper assembly at the end of the coaxial waveguide will excite the associated branch guides 18 equally and in phase. The power emerging on the branch guides is combined at the successive E plane junctions in the converse of the manner in which it was divided in the lower assembly, and arrives at the upper main rectangular guide 4 substantially as supplied to the lower main rectangular guide 4.

It will be apparent without further illustration that either or both coaxial waveguide sections 22, 23 may be extended as necessary to place the assemblies 1 and 3 at separate locations, for example, at the bottom and top respectively of a mast. The coaxial waveguide itself may be used as a mast, or a structural component thereof. In such event, it would usually be preferable to place the bearing assembly 6 at the upper end, i.e., to extend the lower coaxial waveguide to the required length. The inner conductor 23 may be hollow as shown, and additional waveguides or transmission lines may be run through it to connect other equipment without mechanical or electrical interference with the described system. A plurality of rotary joints may be stacked in this way for either parallel or independent operation.

The power that can be handled by the coaxial waveguide in atmosphere without breakdown is greater than the capacity of one of the associated rectangular guides under equivalent conditions by approximately the ratio of the mean circumference of the coaxial waveguide to the narrow dimension of the rectangular waveguide. The placement of pressure windows in the rectangular guide feed system, to permit pressurization of the feed, will determine the degree to which this capability is utilized. Suitable windows may be placed at the flanges connecting each section 18 to its preceding junction, or may be provided in the rectangular openings at the ends of the axially extending sections 20, for an increase of eight times in power handling capacity.

A rotary joint as described above, using rectangular waveguide components of standard S band dimensions (3.0 x 1.5), exhibit a VSWR of less than 1.25 over the range of 2490 to 2965 megacycles per second, an insertion loss of less than 0.15 db, and a transmission wow, i.e. variation of transmission due to spurious modes, with rotation of the joint, of about 0.1 db.

I claim:

1. An around-the-mast type rotary joint for transferring electromagnetic wave energy between a first main rectangular waveguide that is adapted to be coupled to a source and a second rectangular waveguide adapted to be coupled to a load, said second main waveguide being rotatable with respect to said first waveguide about an axis, comprising: a coaxial waveguide having cylindrical inner and outer conductors coaxial with said axis, a first mode conversion device including a plurality of substantially identical ree tangular waveguide sections each including a portion extending parallel to said axis and terminating in a common plane adjacent an end of said coaxial waveguide and perpendicular to said axis, the broad walls of said portions being parallel to respective equiangularly spaced radii from said axis, means coupling each of said rectangular waveguide sections to a respective sector of an annular region conformal in cross section to the space between the inner and outer conductors of said coaxial waveguide, said rectangular waveguide sections each including a further portion extending outwardly from said axis, means including a plurality of symmetrical waveguide junctions interconnecting said waveguide sections electrically in series with each other to said first main rectangular waveguide, all of said interconnecting means and waveguides, except said portions extending parallel to said axis, being disposed at distances from said axis exceeding the radius of said cylindrical outer conductor of said coaxial waveguide; a second mode conversion device.

substantially identical to said first mode conversion device, disposed in similar relationship to the other end of said coaxial waveguide and connected to said second rectangular waveguide, and rotatable therewith about said axis, said second mode conversion device being axially spaced from the adjacent end of said coaxial waveguide to provide circumferential gaps between the ends of the conductors of said coaxial waveguide and the respective portions of said mode conversion device.

2. The invention set forth in claim 1, wherein said inner conductor of said coaxial waveguide is hollow, further including a second rotary joint disposed coaxially with and beyond said first rotary joint, and waveguide disposed within said inner conductor throughout a major portion of its length, said last mentioned waveguide being connected to the first main rectangular waveguide associated with said second rotary joint.

3. The invention set forth in claim 1, wherein each of said symmetrical waveguide junctions is an E plane junction.

4. The invention set forth in claim 1, wherein said rectangular waveguide sections are gas filled to provide power handling capability approximating that of said coaxial waveguide.

5. A waveguide structure for electromagnetic wave conversion between the rectangular TE mode and the circular coaxial TE mode, comprising a first terminal section of rectangular waveguide for connection to air exter- 3O nal main rectangular waveguide, a second terminal section of coaxial waveguide for connection to an external main coaxial waveguide, a first rectangular waveguide E plane junction connecting said first terminal waveguide section symmetrically to two rectangular branch waveguides, said branch waveguides being equal in length, second and third rectangular waveguide E plane junctions connecting each of said branch waveguides symmetrically to a further respective pair of branch waveguides, and a plurality of further waveguide junctions and further branch waveguides similarly connected to form an assembly terminating in 2 branch waveguide open ends where n is an integer, said branch waveguide ends being disposed in a plane facing an end of said second terminal section and perpendicular to the axis thereof, with their broad dimensions parallel to respective equiangularly spaced radii of said coaxial waveguide terminal section, and wedge shaped conductive members lying between and interconnecting the broad walls of adjacent branch waveguide ends, said members being of reduced angular thickness toward said end of said coaxial waveguide terminal section to provide transitions between said branch waveguide ends and respective sectors of an annular region conforming to the cross section of said coaxial waveguide section.

6. The invention set forth in claim 5, wherein all of said rectangular waveguide elements are nominally identical in cross section, and each of said E plane junctions includes transformer means for matching each waveguide to its respective branches.

7. The invention set forth in claim 5, wherein the broad wall dimension of said rectangular waveguides is approximately equal to the difierence between the radii of the outer and inner conductors of said coaxial waveguide.

References Cited by the Examiner UNITED STATES PATENTS 2,628,311 2/53 Lindenblad 333-9 2,975,382 3/61 Fromm 333-98 3,127,5'79 3/64 Smith 333-98 OTHER REFERENCES Fromm, W. E., et al.:-IRE National Convention, Part l, A New Microwave Rotary Joint, pages 78-82, vol.

HERMAN KARL SAALBACH, Primary Examiner. 

1. AN AROUND-THE-MAST TYPE ROTARY JOINT FOR TRANSFERRING ELECTROMAGNETIC WAVE ENERGY BETWEEN A FIRST MAIN RECTANGULAR WAVEGUIDE THAT IS ADAPTED TO BE COUPPLED TO A SOURCE AND A SECOND RECTANGULAR WAVEGUIDE ADAPTED TO BE COUPLED TO A LOAD, SAID SECOND MAIN WAVEGUIDE BEING ROTATABLE WITH RESPECT TO SAID FIRST WAVEGUIDE ABOUT AN AXIS, COMPRISING: A COAXIAL WAVEGUIDE HAVING CYLINDRICAL INNER AND OUTER CONDUCTORS COAXIAL WITH SAID AXIS, A FIRST MODE CONVERSION DEVICE INCLUDING A PLURALITY OF SUBSTANTIALLY IDENTICAL RECTANGULAR WAVEGUIDE SECTIONS EACH INCLUDING A PORTION EXTENDING PARALLEL TO SAID AXIS AND TERMINATING IN A COMMON PLANE ADJACENT AN END OF SAID COAXIAL WAVEGUIDE AND PERPENDICULAR TO SAID AXIS, THE BROAD WALLS OF SAID PORTIONS BEING PARALLEL TO RESPECTIVE EQUIANGULARLY SPACED RADII FROM SAID AXIS, MEANS COUPLING EACH OF SAID RECTANGULAR WAVEGUIDE SECTIONS TO A RESPECTIVE SECTOR OF AN ANNULAR REGION CONFORMAL IN CROSS SECTION TO THE SPACE BETWEEN THE INNER AND OUTER CONDUCTORS OF SAID COAXIAL WAVEGUIDE, SAID RECTANGULAR WAVEGUIDE SECTIONS EACH INCLUDING A FURTHER PORTION EXTENDING OUTWARDLY FROM SAID AXIS, MEANS INCLUDING A PLURALITY OF SYMMETRICAL WAVEGUIDE JUNCTIONS INTERCONNECTING SAID WAVEGUIDE SECTIONS ELECTRICCALLY IN SERIES WITH EACH OTHER TO SAID FIRST MAIN RECTANGULAR WAVEGUIDE, ALL OF SAID INTERCONNECTING MEANS AND WAVEGUIDES, EXCEPT SAID PORTIONS EXTENDING PARALLEL TO SAID AXIS, BEING DISPOSED AT DISTANCES FROM SAID AXIS EXCEEDING THE RADIUS OF SAID CYLINDRICAL OUTER CONDUCTOR OF SAID COAXIAL WAVEGUIDE; A SECOND MODE CONVERSION DEVICE SUBSTANTIALLY IDENTICAL TO SAID FIRST MODE CONVERSION DEVICE, DISPOSED IN SIMILAR RELATIONSHIP TO THE OTHER END OF SAID COAXIAL WAVEGUIDE AND CONNECTED TO SAID SECOND SECTANGULAR WAVEGUIDE, AND ROTATABLE THEREWITH ABOUT SAID AXIS, SAID SECOND MODE CONVERSION DEVICE BEING AXIALLY SPACED FROM THE ADJACENT END OF SAID COAXIAL WAVEGUIDE TO PROVIDE CIRCUMFERENTIAL GAPS BETWEEN THE ENDS OF THE CONDUCTORS OF SAID COAXIAL WAVEGUIDE AND THE RESPECTIVE PORTIONS OF SAID MODE CONVERSION DEVICE. 